In stringed instruments, amplification of low amplitude string vibrations is usually accomplished by utilizing an arrangement of wire-wound magnets under the strings of the instrument. To produce sound, the wire-wound magnets sense the vibrations of the strings electronically and a signal cable routes an electronic signal to an amplifier and speaker. The sensing occurs in a magnetic pickup mounted under the strings on the guitar's body. A bar magnet is typically used. When a vibrating string cuts through the field of the bar magnet in the pickup, a signal is produced in the pickup's coil. This pickup consists of a bar magnet wrapped with as many as 7,000 turns of fine wire. In the case of an electric guitar, the vibrating steel strings produce a corresponding vibration in the magnet's magnetic field and therefore a vibrating current in the coil.
There are many different types of pickups. For example, some pickups extend a single magnet bar under all six strings. Others have a separate pole piece for each string. Some pickups use screws for pole pieces so that the height of each pole piece can be adjusted. The closer the pole piece is to the string, the stronger the signal.
The pickup's coil sends its signals through a very simple passive component circuit on most guitars. A potentiometer adjusts the tone along with a low-pass filter that eliminates higher frequencies. By adjusting the potentiometer, it is possible to control the frequencies that get cut out. An additional resistor (typically 500 kilo-ohms max) controls the amplitude of the signal that reaches the jack. From the jack, the signal runs via the signal cable to an amplifier, which drives a speaker.
The present invention is a device that uses an integrated system of electronics and at least one bipolar magnet that is packaged on a thin and compact plastic tray that can be slipped between a guitar's strings and the body of the guitar. The device requires no holes be cut into the guitar and includes a volume control, output jack and an off-on battery switch on the tray. The device typically works with batteries that provide direct current and the entire device can be unplugged and removed if the player wants to play the guitar acoustically. The bipolar magnets may be placed anywhere on the tray as long as they are located near, or preferably under the guitar strings. As the magnets may be moved to different locations near the strings, the change in location of the magnets causes the various tones to be available to the player. If the magnets are moved rapidly, tremolo effects can be heard. The magnets may be of virtually any shape, although it is preferred that the magnets remain small and compact in size.
The strings are coupled to digital electronic processing for amplifying the string current that is induced from the bipolar magnets while eliminating annoying audio noise, principally Gaussian and 60 Hz hum. By utilizing digital processing, one can selectively boost or cut frequencies and further reduce noise from the string signal that were not possible with analog filters as described above.
The amplifiers used in the preferred embodiment are high-precision differential amplifiers that have high common mode rejection of about 90 db or more and low output noise of about 10 nano volt root hertz or less. High CMR is achieved by use of laser trimming of internal resistors in the differential amplifier IC to perfect matching ratio, which provides very high CMR.
It is also desirable to utilize super high field strength neodymium magnets of at least grade ND40. By using the high field strength magnets, a high string current is induced and consequently, much of the common noise found with conventional guitar string pickups is rejected. The present invention can also be used with all shapes and sizes of neodymium magnets. Typically, these types of magnets are economical because the manufacturing process simply involves the use of molds filled with neodymium powder and compressed into that mold shape.